Because of their ability to transmit larger quantities of information over a transmission line than conventional systems, optical communications systems are coming into increasingly widespread use. In its simplest form, such a system comprises an optical source such as a semiconductor laser that can be modulated with information, an optical fiber for transmitting the modulated lightwave, and a detector such as a semiconductor photodetector for converting the optical information back into electrical information.
Both semiconductor lasers and photodetectors are typically made of semiconductor material and as such are technically related to devices commonly used in the electronics industry. It is well understood that such devices must ordinarily be sealed from the environment to protect them from atmospheric contaminants, particularly water vapor. In the electronics industry, a number of encapsulants have been developed that have been found to be suitable for this purpose; for example, various silicone elastomers, and various epoxies provide good protective coatings. The semiconductor encapsulation technology cannot, however, be transferred directly to optical communications systems because of the need for the encapsulations to be transparent at the wavelengths used. For this reason, it is customary in optical communications systems to use packages for protecting semiconductor optical devices such as lasers and photodetectors, such packages each containing a glass window for permitting light to be transmitted to or from the optical device. Such packages are bulky and expensive and a great deal of effort has been directed toward reducing the bulk and expense associated with hermetic packages for semiconductor optical devices that depend upon light transmission for their function.